The commercial fabrication of semiconductor chips is done in the form of wafers which are typically four to eight inches in diameter. Usually, there are more than 100 processing steps in the fabrication which include oxidation, diffusion, ion implantation, deposition of conductors and insulators, photolithography, and etching. Various conducting and insulating layers are deposited uniformly over the wafer to a thickness of a few microns. Photolithography, in a subtractive process, defines areas within the deposited layers which are to be retained by protecting them with an etch resistant layer, and exposing areas which are to be removed by etching.
Two trends in semiconductor processing technology make the etching step more critical. First, the increased functionality of each chip requires a greater packing density within each chip and therefore finer lines for conductors and smaller openings in insulating layers. Second, increased reliability of semiconductor circuits is achieved with materials that are chemically impervious and also resistant to diffusion. Conductors are often formed from refractory metal silicides, such as the silicides of tantalum or titanium. Insulating layers are typically formed from silicon dioxide or silicon nitride. These layers provide excellent insulation and also serve to prevent the diffusion into the chip of harmful contaminants.
The fine features in the circuit and the chemical inertness of the layers preclude wet chemical etching which produces a tapered edge in a feature. The width of some features is comparable to the thickness of the film from which they are etched. Any taper or lack of control in producing the taper severely limits the ratio of feature width to thickness.
These problems are overcome by the use of radio frequency (rf) energy to create a plasma of extremely reactive ions within an etching chamber of an etching machine. The interior of the chamber is evacuated to remove atmospheric gases and water vapor. Selected gases are then admitted into the chamber at a controlled flow rate. The gases typically contain chlorine or fluorine atoms, and they may be extremely inert in the atmosphere. However, these gases become ionized in the presence of rf energy at the reduced pressure in the etching chamber. The ions have the ability to remove silicon oxide or silicon nitride, and to do so with much less taper in the wall of a feature than with conventional wet chemical etching. A variation of plasma etching, called reactive ion etching, causes the active ions to impinge upon the layer with a momentum. This further reduces the taper.
Production in-line plasma etching machines, for example, a machine marketed under the trade name "Plasma-Therm, Model A360 or A368", are available. A series of semiconductor wafers are placed in an input chamber of the etching machine which is evacuated with a vacuum pump. Each wafer is then transported in turn to an etching chamber of the etching machine and placed on an electrode contained therein. A gas, or a mixture of gases, is ionized by rf energy to form a plasma within the chamber. The rf energy is only supplied if the sensors within the machine indicate that a wafer is present on the electrode which supplies the energy. The chamber walls are grounded. The ions combine with areas of the top layer of the wafer which are not protected with a photographically defined masking layer (e.g., silicon nitride). Most of the by-products of this reaction, and the by-products of a lesser reaction with the masking layer, are pumped away by a vacuum pump which maintains a constant pressure within the etching chamber. However, some of these by-products deposit as a solid polymer on interior walls of the chamber and upon the electrode supporting the wafer.
Typically semiconductor wafers are circular with one or more flats ground on the circumference. These flats serve to identify the crystallographic orientation of the wafer. The random placement of many wafers on the supporting electrode and the etching of each ultimately causes the formation of an annular ring of polymer near the outer edge of the electrode where the electrode is not covered by the flat of the wafer.
After many wafers have been etched, the polymer deposit on the electrode outer circumference supporting the wafer becomes thick enough to interfere with the wafer's contact with the electrode. This results in non-uniform etching across the wafer as well as missed transfers due to a wafer sticking to polysilicon buildup on the electrode. Non-uniformity exceeding seven percent is beyond some specified limits in turn affecting side wall profile variance across the wafer. Experience in some fabrication facilities has shown that this level is reached after 100 to 150 wafers have been processed. It is then necessary to vent the chamber and remove the annular ring of polymer with a manual solvent wiping operation. The chamber must then be requalified before returning to production. This causes considerable down-time and also raises the possibility of contamination entering the chamber when it is exposed to the atmosphere.
It is desirable to clean the build up of polymer deposits on interior walls of an etching chamber and the polymer ring formed on the electrode of an etching machine, such as the aforementioned Model A360, without opening the etching chamber. This would increase production efficiency and reduce the possibility of contamination from the atmosphere.